The invention relates to a method and apparatus for purifying a gas flow.
Although the present method and apparatus can be applied for purifying gases in environments of very different character, the present method and apparatus are especially well suited for purifying exhaust gases of internal combustion engines, preferably diesel engines.
One objective in environmental protection is to reduce exhaust emissions of combustion engines. The present invention provides a solution for reducing the exhaust emissions of internal combustion engines.
Diesel engine exhaust emissions contain various harmful combustion products. They include gaseous carbon monoxide, which oxidises rapidly and forms carbon dioxide that contributes to the greenhouse effect, and volatile organic compounds, solid compounds, and particulates. When heavy oil burns, sulphur oxides SOx, typically SO2 and SO3, are produced, which contribute to the acidification of soil and the production of smog. During the combustion process, some of the nitrogen gas present in the air oxidises to nitrogen oxides NOx, particularly NO and NO2, which strongly contribute to the formation of smog, to the greenhouse effect and the acidification of soil and to retardation of forest growth, for example. In addition, a fuel that contains nitrogen may cause formation of nitrogen oxides.
The amount of sulphur emissions from diesel engines can be reduced by using low-sulphur fuel: the less sulphur the fuel contains, the lower are the sulphur emissions. Low-sulphur fuel can be produced in refineries, but at a high cost. The amount of sulphur oxides cannot be influenced by making changes to the combustion process.
Diesel engines of ships are major air polluters. According to a study made in the United States a few years ago, 14% of the world nitrogen emissions and 16% of sulphur emissions of oil exhaust gases originate from shipping. Nitrogen emissions from diesel engines are created most when the motor runs at a low power. In ships this typically takes place when the ship is in port, which is naturally regrettable.
The amount of nitrogen emissions of diesel engines can be reduced by decreasing the combustion temperature. The temperature can be decreased either by adding water into the fuel or by spraying an emulsion containing water into the combustion chamber. By decreasing the combustion temperature, the amount of nitrogen oxide emissions can be reduced by about 10%. A decrease in the combustion temperature reduces the efficiency of the engine.
Nitrogen emissions can be efficiently removed from exhaust gases with a catalytic converter in which a mixture of urea and water is sprayed into the fuel gases to reduce nitrogen oxides to nitrogen and water vapour. However, in marine applications, catalytic converters are very expensive; they account for about 30% of the engine cost, and even more in large ships. In addition, a catalytic converter requires servicing, and a space needs to provided for it in the ship.
U.S. Pat. No. 5,464,458 teaches a system in which diesel engine exhaust gases are purified using mist-like water. The system employs nozzles to spray water into a chamber containing a flow of exhaust gases to separate products contained in the exhaust gases from them. The system allows carbon in particular, but also carbon monoxide, hydrocarbons and nitrogen oxides, to be removed from the exhaust gases.
It is an object of the present invention to provide a new, economical generally applicable solution for purifying a gas flow. The solutions is especially well suited for purifying exhaust gas emissions of internal combustion engines and especially large-dimension both two-stroke and four-stroke diesel engines, especially diesel engines of ships, the solution being implemented such that it does not complicate the discharge of exhaust gases from the engine.
To achieve this objective, the invention provides a method for purifying a gas flow, the method comprising spraying of water mist into the gas flow by utilizing at least one spray nozzle thereby causing a negative pressure in the immediate vicinity of the nozzle for sucking the gas flow into the sprayed spray. The sucking causes gas to be entrained into the sprayed spray.
The water mist is preferably created by a pressure of 10 to 300 bar and if applied to purifying exhaust gases of an internal combustion engine the spray is sprayed inside the exhaust manifold of the engine, and the engine, and the water mist is preferably sprayed in an at least approximately parallel direction with the exhaust gas flow.
Preferably at least part of the gases to be purified are deviated out of a principal flow path by the suction provided by the spray head.
To achieve the above-mentioned objective, the invention provides a apparatus comprising a support structure for holding at least one spray nozzle, the spray nozzle being arranged to spray water mist into the gas flow, wherein the spray nozzle is a spray nozzle the operation of which creates a suction causing a negative pressure in the immediate vicinity of the nozzle. The negative pressure draws the gases toward the spray nozzle and guides the gases into the water spray coming from the spray nozzle. This allows for an efficient reaction between the gases and water.
The spray nozzle is preferably arranged inside a reaction chamber which comprises an inlet opening for directing a flow of gases into the suction of the spray nozzle inside the reaction chamber.
Preferred embodiments of the apparatus of the invention are disclosed in the accompanying claims.
Major advantages of the present invention for purifying a gas flow are that it is fairly simple and inexpensive to implement; the invention can be applied to existing systems, included internal combustion engines, such as diesel engines of ships, and therefore only minor changes are needed for said implementation; the method and apparatus allows to guide the gases to be purified in desired direction. When applied to an internal combustion engines, the apparatus provides for an efficient, even improved, removal of exhaust gases from the engine. Moreover, the operating costs of the apparatus, with service costs included, are very low.